A breaking apparatus generally called a concrete breaker is used for breaking a hard object such as asphalt or concrete at a road construction site or a building construction site, for example. In a typical breaker, an engine mounted at an upper portion of the main body drives a striker. The striker, when driven, causes a work member supported by a lower portion of the main body to move up and down to break the hard object. FIG. 9 illustrates an example of prior art engine breaker. The engine breaker 100 includes a vertically extending cylindrical main body 1, a work member 2 supported by a lower portion of the main body for up and down movement, an engine 105 mounted to an upper portion of the main body 1, and a striker 30 incorporated in the main body 1 for successively striking an upper end of the work member 2 by the driving of the engine 105.
The main body 1 includes a cylindrical member 11, and a crank case 12 mounted to an upper portion of the cylindrical member 11. The cylindrical member 11 and the crank case 12 incorporate a hammer 3 and a crank mechanism 4, respectively, which will be described later. The work member 2 may be a chisel having a drill-like tip end, and a base end inserted in the cylindrical member 11.
The engine 105 is so arranged that the output shaft thereof extends horizontally and transmits the rotation output to a rotary shaft 6 incorporated in the crank case 12 of the main body 1. The rotary shaft 6 is so arranged in the crank case 12 as to extend horizontally. The engine 105 is mounted to the main body 1 via an engine mount plate 7a having an L-shaped cross section. Specifically, a side surface and a lower surface of a housing of the engine 105 are screwed to the engine mount plate 7a, and the engine mount plate 7a in this state is screwed to a side surface of the crank case 12 of the main body 1. As the engine 105, a two-cycle engine with a displacement of about 50 cc is generally used.
The striker 30 includes a crank mechanism 4 operated by the rotation of the rotary shaft 6, and a hammer 3 which moves up and down by the operation of the crank mechanism 4. The hammer 3 has a front end for contacting the work member 2.
Unlike a breaker in which a hammer 3 moves up and down by utilizing an expansion force of compressed air, the engine breaker 100 is not connected to an external apparatus such as a compressor for generating compressed air, and hence is relatively easy to handle.
In the engine breaker 100, however, the main body 1 vibrates much due to the reaction in moving the hammer 3 by the crank mechanism 4 and the impact in striking the work member 2 with the hammer 3. As a result, the housing of the engine 105 mounted to the main body 1 may be deformed at a portion close to the main body 1 or the engine 105 itself may vibrate to result in a change in the positional relationship between the structural parts of the engine disposed inside or outside of the housing. Such a condition may lead to the malfunction or failure of the engine 105. Therefore, to enhance the durability of the engine breaker 100, the wall thickness of the housing of the engine 105 may be made to about 5 to 6 mm, which is considerably larger than the wall thickness (2 to 3 mm) of a general two-cycle engine with a displacement of about 50 cc. Therefore, such a general engine cannot be used as the engine 100, which leads to an increase of the manufacturing cost of the engine breaker 100. Further, the weight of the engine 105 having a housing made of diecast aluminum becomes about 8 kg, which is considerably larger than the weight (about 3 kg) of a general engine. In this way, the large wall thickness of the housing makes it difficult to reduce the weight of the engine breaker 100.